Nicene and Post-Nicene Fathers, Vol. IV:
Writings in Connection with the Donatist Controversy.: Chapter 7

Chapter 7.—8.  Truly, when I look at the actual words of Cyprian, I am warned to say some things which are very necessary for the solution of this question.  "For if they were to see," he says, "that it was settled and established by our formal decision and vote, that the baptism with which they are baptized in heresy is considered just and lawful, they will think that they are in just and lawful possession of the Church also, and all its other gifts." 1475   He does not say "that they will think they are in possession," but "in just and lawful possession of the gifts of the Church."  But we say that we cannot allow that they are in just and lawful possession of baptism.  That they are in possession of it we cannot deny, when we recognize the sacrament of the Lord in the words of the gospel.  They have therefore lawful baptism, but they do not have it lawfully.  For whosoever has it both in Catholic unity, and living worthily of it, both has lawful baptism and has it lawfully; but whosoever has it either within the Catholic Church itself, as chaff mixed with the wheat, or outside, as chaff carried away by the wind, has indeed lawful baptism, but not lawfully.  For he has it as he uses it.  But the man does not use it lawfully who uses it against the law,—which every one does, who, being baptized, yet leads an abandoned life, whether inside or without the Church.

Footnotes

Ib.